Image readers are known in which an image of an original color document is formed on a solid state image pickup element such as a CCD (Charge Coupled Device) via an optical system in order to read image information from the original color document. Recently, such image readers also enable reading film originals because of the remarkable progress made in providing high resolution in image readers, such as image scanners.
On the other hand, unnecessary image information such as very fine scratches and dust on a film surface are read along with the image as part of the original target information when high resolution imaging is used. Therefore, attempts have been made to detect the unnecessary image information, such as scratches and dust, by irradiating near-infrared light on the film surface and removing this unnecessary image information by electronic image processing. Accordingly, a lens that enables high resolution reading not only in the visible region but also in the near-infrared region is desired as an image scanning lens used in image readers including such a feature. Examples of such reading lenses that operate as imaging optical systems are disclosed in Japanese Laid-Open Patent Application 2002-148514 and Japanese Laid-Open Patent Application 2004-29474.
However, the imaging optical systems disclosed in the Japanese applications mentioned above have a relatively small field angle. Although this is acceptable for reading smaller film such as thirty-five millimeter film, there is a need to upgrade the lens in order to read larger film and reflecting originals (for example, about A4 size), which cannot be done with these imaging systems. Therefore, an image scanning lens having a larger field angle is desired, as well as an image scanning lens that has a smaller size and a smaller number of lens elements.